1. Field of the Invention
The present invention relates to a laser beam light source used to write an image by means of light beams in a digital copying machine, a laser beam printer and the like and, in particular, to a multibeam semiconductor laser array serving as a light source which is able to perform simultaneous image writing by a plurality of laser beams and a multibeam laser printer which is constructed by use of such a multibeam semiconductor laser array.
2. Description of the Related Art
In a laser beam printer, a laser beam from a semiconductor laser is irradiated on a rotary polyhedral mirror called a polygon scanner, and the beam reflected from the rotary polyhedral mirror is then irradiated on the surface of a charged photoreceptor which is moving at a constant speed. Rotation of the rotary polyhedral mirror causes the laser beam to scan in a direction perpendicular to the moving direction of the photoreceptor. Since the laser beam is modulated according to the image to be output, there is formed an electrostatic latent image on the photoreceptor and the electrostatic latent image is developed to provide a visible toner image.
However, in the above-mentioned laser beam printer, distances between the scanning lines of the laser beam must be set narrow enough in order to enhance the fineness of an output image. Also, in order to output the image at a high speed, the scanning speed must be increased. The most important problem in improving the image fineness and speed of the laser beam printer is that the rotatory speed of the polygon scanner is limited.
In order to solve the above-mentioned problem, a multibeam scanning system which scans on an image plane at the same time by means of a plurality of laser beams is already known. In this multibeam scanning system, of course, a plurality of laser beam spots must be sufficiently near to each other in a direction (which will be hereinafter referred to as a subsidiary scanning direction) perpendicular to a direction of scanning (which will be hereinafter referred to as a main scanning direction) by a polygon scanner. For this purpose, efforts have been made to manufacture a plurality of semiconductor lasers such that they are near to each other. At present, a semiconductor laser array is made on an experimental basis in which the semiconductor lasers are closely spaced down to a distance of 10 .mu.m (see, for example, Japanese Patent Unexamined Publication No. Hei. 2-39583, R. L. Thornton et al., "Properties of closely spaced independently addressable lasers fabricated by impurity-induced disordering", Appl. Phys. Lett. 56(17), 1623-1625 (1990) and the like).
However, even if the plurality of semiconductor lasers are closely spaced around 10 .mu.m by employing the techniques respectively disclosed in the above-mentioned Japanese Patent Unexamined Publication and the like, such distance is not sufficient to scan leaving no spaces in the subsidiary scanning direction. As means for making up for such insufficiency, there has been proposed a method which can scan the above-mentioned unscanned spaces in the subsidiary scanning direction by means of interlaced scanning (see Japanese Patent Examined Publication No. Hei. 1-45065). Also, there has been proposed a multibeam scanning optical system using a semiconductor laser array including a plurality of semiconductor lasers spaced 10 .mu.m from each other.
In FIG. 1, an example of interlaced scanning is shown. In this example, interlaced scanning is carried out by two laser beams L1 and L2. In FIG. 1, d.sub.x designates a laser spot diameter to be defined electrophotographically (which is hereinafter referred to as an electrophotographic spot diameter). The electrophotographic spot diameter is not a optically defined laser spot diameter existing on an image plane A1, but it means a diameter of a spot occurring when an electrostatic latent image formed on the image plane A1 by a laser is developed. A distance r.sub.3 between the centers of two spots B1 and B2 formed on the image plane A1 by the two laser beams L1 and L2 is 3d.sub.x.
Since subsidiary scanning is performed by 2d.sub.x in each main scanning, as shown in FIG. 1, in the first scanning a second scanning line is scanned by the laser beam L2, and in the second scanning a first scanning line is scanned by the laser beam L1 and a fourth scanning line is scanned by the laser beam L2. In this manner, the following scanning lines are scanned sequentially without leaving any scanning lines between them. In other words, although some of scanning lines are left unscanned in each scanning, the line scanned in a certain scanning time is jumped in the next scanning time, so that all of the scanning lines are scanned with no lines left unscanned in the long run.
In order to prevent duplicate scanning and to prevent occurrence of unscanned lines in the interlaced scanning, the following three conditions must be satisfied:
1) The subsidiary scanning must be executed by nd.sub.x with respect to the number of laser beams n in one main scanning.
2) The distance r.sub.3 between the two laser beams on the image plane must be an integral multiple of the electrophotographic spot diameter d.sub.x.
3) When an integer obtained by dividing the spot center distance r.sub.3 by the spot diameter d.sub.x is called an interlacing period I, the interlacing period I must be mutually prime to the number of laser beams n (the greatest common measure between them is 1).
Further, the smallest distance between the scanning lines is referred to as a scanning line pitch which is designated by p in FIG. 1. In general, p=d.sub.x. The spot diameter of the laser beam is generally defined by a diameter which allows the light amplitude to be 1/e (1/e.sup.2 in power) of that of the spot center. The thus defined spot diameter is hereinafter referred to as an optical spot diameter and is designated by d.sub.o.
In the laser beam printer, an image is formed according to an electrophotographic process. However, when the image obtained according to the electrophotographic process is examined, it is preferred to define a spot diameter which is different from the above-mentioned optical spot diameter. That is, a spot diameter of an image finally obtained when a laser beam having a certain light intensity distribution is used is defined as the electrophotographic spot diameter d.sub.x of the laser beam.
In FIG. 2, there is shown a relationship between the optical spot diameter and the electrophotographic spot diameter. In this figure, the light amplitude on the main light ray of the laser beam is normalized to 1. While the optical spot diameter d.sub.o is a diameter in which the light amplitude is 1/e (1/e.sup.2 in power) of that of the spot center, the electrophotographic spot diameter d.sub.x is a diameter in which the light amplitude is x times of that of the spot center (x.apprxeq.0.7, when converted to a power ratio, approximately 0.5) (see "Examination on Tone Reproduction in Laser Xerography" proposed by Tanaka, the 6th Color Engineering conference, p. 77-p. 80 (1989)) Here, a ratio of the optical spot diameter d.sub.o to the electrophotographic spot diameter d.sub.x is referred to as a spot diameter correction coefficient k, and k is defined as follows: EQU k=d.sub.o /d.sub.x
The value of k varies according to the processes of electrophotography used. In a process of reversal development in which a toner is attached to a light exposed area, 1.4.ltoreq.k.ltoreq.1.6 is desirable. On the other hand, in a process of forward development in which the toner is attached to a light exposed area, 1.5.ltoreq.k.ltoreq.1.8 is desirable.
The laser beam printer basically employs an image forming method based on the electrophotographic process and, of course, a light source used in the laser beam printer must be able to allow a photoreceptor for electrophotography to emit a wave length having a practical sensitivity. In recent years, compact disc players have been mass produced so that the costs of AlGaAs semiconductor lasers having a light emitting wave length of 0.78 .mu.m used in the compact disc players have been reduced. Due to this, the development of photoreceptors for electrophotography in conformity with the above light emitting wave length has been advanced and at present photoreceptors having sufficient performance are being practically used. Partly because a photoreceptor having a sensitivity to a light with a long wave length is generally apt to be unreliable, at present, it can well be said that there exist almost no photoreceptors which have a practical sensitivity to the light having a wave length longer than 0.78 .mu.m. From this point of view, it can be said that a semiconductor laser used in the laser beam printer should be able to emit a light having a wave length shorter than approximately 0.8 .mu.m. Practically, it is desired to emit a light having a further shorter wave length and thus an AlGaInP semiconductor laser which is able to emit a light with a wave length of the order of 0.68 .mu.m is more suited for the laser beam printer.
In the semiconductor laser array to be used in the above-mentioned applications, however, a plurality of semiconductor laser elements are disposed very closely to each other and, therefore, it is difficult to arrange collimators individually with respect to each semiconductor laser element. From this point of view, every laser beam light must be focused on an image plane with the same optical system. Using the optical configuration, however, it is almost impossible to independently change a spot diameter and a spot distance on the surface to be scanned. This makes it critical for optical conditions to be set in such a manner that a proper interlaced scanning can be performed.
Also, it seems that, according to the above-mentioned interlaced scanning, even if the image forming spot distance is set to be wide on the image plane, no problem arises provided that a proper interlacing period is selected. However, in fact, if the image spot distance is set to be wide and a great interlacing period (which is hereinafter referred to as an interlacing period of higher order) is employed, then very high mechanical precision is required of a scanning device. Description will be given below of reasons for such very high mechanical precision:
As shown in FIG. 3 (a), when the image plane is scanned by a spot B, that is, when the number of semiconductor laser elements n=1 and the interlacing period I=1, if it is assumed that a scan pitch p must be within a certain error .DELTA.p, then a tolerance .delta..sub.o of a speed in the subsidiary scanning direction can be expressed by the following equation: EQU .delta..sub.o =.DELTA.v/v=.DELTA.p/p
where v and .DELTA.v denote the subsidiary scanning speed and the error of v, respectively.
FIG. 3 (b) shows a case in which the number of semiconductor laser elements n=3 and the interlacing period I=2. Assuming that a tolerance of the speed is .delta..sub.n,I when the number of semiconductor laser elements is expressed as n and the interlacing period is expressed as I, then the tolerance of the speed in this case, .delta..sub.3,2 =.DELTA.p/3p=.delta..sub.o /3. Accordingly, the tolerance of the speed is reduced to 1/3 when compared with that obtained in the case of FIG. 3 (a).
As shown in FIG. 3 (c), when the number of semiconductor laser elements n=4 and the interlacing period I=5, then there is obtained the tolerance .delta..sub.4,5 =.DELTA.p/16p=.delta..sub.o /16. This means that the required a precision increases more than one decade compared with the case shown in FIG. 3 (a).
In the above discussion, description has been given of the tolerance of the speed in the subsidiary scanning direction. Besides the tolerance of the speed in the subsidiary scanning direction, similar problems occur in the accuracy of the optical system and the dimensional accuracy of the semiconductor laser array. In addition, when the image forming spot distance is set to be wide and a high order interlacing period is employed, then the required number high speed memories, which are expensive, to control the interlaced scanning are increased.